The present invention relates to a loop-gain control system in spindle servo loop for controlling the rotating speed of a disc and particularly relates to a loop-gain control system in a spindle servo loop for playing a so-called CLV disc.
A CLV disc is a disc on which recording has been made so that the linear speed is constant in playback, and the defined rotating speed of the disc changes correspondingly to the relative position of the pickup in the radial direction of the disc as shown in FIG. 4. In the case where playback starts from a desired section or chapter on a CLV disc even if the spindle servo for controlling the rotating speed of the disc locks-in once, it still takes time for the spindle servo to lock-in again after the pickup has been moved to the desired chapter resulting in a problem that it takes a long time before starting playback. Therefore, it has been proposed to turn on the spindle servo after the pickup has been moved to a center portion of a recording area. In such case, since the lock-in of the spindle servo can be obtained at a lower rotating speed than that in the innermost circumference, the time taken for lock-in can be shortened, and since the distance to a desired chapter is shortened because of averaging the time taken for movement of the pickup is shortened, so that the time required before starting playback can be shortened.
Such a conventional disc player has used a system in which a time code read by the pickup constitutes address data indicating the recording position, the relative position of the pickup in the radial direction of the disc is detected, and the defined rotating speed of the disc is calculated so that control of the loop gain of the spindle servo can be performed in accordance with the rotating speed of the disc on the basis of the calculated value of the defined rotating speed.
In such a conventional system however, there has been a disadvantage if the time code recorded on the CLV disc does not start from 0 seconds. An error may occur in the calculation of the defined rotating speed so as to make the loop gain control of the spindle servo erroneous thereby making it impossible to perform a search or the like.
In order to eliminate this problem, it has been proposed to provide a potentiometer on a slider for mounting the pickup thereby producing a position detection signal corresponding to a relative position of the pickup in the radial direction of a disc. The obtained position detection signal performs the control function for switching the loop gain of the spindle servo. In this method, however the number of adjustment steps increases as the number of parts increases so that the production costs rise and reliability suffers.